This invention relates to a method of producing niobium nitride (NbN) films of high superconducting transition temperature (T.sub.c) by dc reactive magnetron sputtering, and more particularly for producing such films on a substrate maintained at room temperature.
NbN thin films are important for a variety of applications due to their high superconducting transition temperature, and their robust refractory nature. They have been used as coatings on carbon fibers for superconducting cables, for superconducting magnets utilizing tape windings [R. T. Kampwirth, et al. ibid., 498 (1985)], and to form superconductor-insulator-superconductor (SIS) tunnel junctions for Josephson device applications [J. C. Villegier, et al., ibid., 498 (1985), E. J. Cukauskas, et al., ibid., 505 (1985), R. B. VanDover, et al., Appl. Phy. Letters 41, 764 (1982)]. Recently, superconductor-insulator-superconductor (SIS) junctions have also attracted a great deal of attention because of their potential use as quasi-particle tunneling based quantum mixers in submillimeter wave heterodyne receivers. [T. G. Phillips, et al., Ann. Rev. Astron. Astrophys., 20 285 (1982), J. R. Tucker, Appl. Phys. Lett. 36 477, (1980), K. H. Gundlach, et al., Appl Phys. Lett. 41, 294 (1982)] The presently used, mechanically soft, Pb-based SIS devices degrade on thermal cycling. Moreover, a low superconducting gap parameter (.about.1.5 meV) limits their use to frequency values .about.700 GHz.
Niobium nitride, with its hard refractory nature, offers a potential of stability over repeated thermal cycling, and its high superconducting gap parameter (.about.3 meV) promises an extended frequency range of application up to .about.1500 GHz. [T. H. Geballe, et al., Physics 2, 293 (1966)] predicted an upper limit of 18.degree. K. for stoichiometric NbN. Since then, numerous workers have deposited NbN thin films by a variety of techniques [J. R. Gavaler, et al., J. Vac. Sci. Tech. 6, 177 (1969), Y. M. Shy, et al., J. Appl. Phys. 44, 5539 (1973), Gin-ichiro Oya, et al., J. Vac. Sci. Tech. 7, 644 (1970), S. A. Wolf, et al., ibid, 17 411 (1980), K. S. Keskar, et al., J. Appl. Phys. 45, 3102 (1974), K. Takei, et al., Jpn. J. Appl. Physics 20, 993 (1981), R. T. Kampwirth, et al., IEEE Trans. Magn. Mag-17, 565 (1981)], and have obtained T.sub.c values in a range of 15.degree.-17.degree. K. using high substrate temperatures (.gtoreq.500.degree. C.). However, for the ease of device fabrication, it is of extreme importance to be able to deposit NbN films on substrates held at room temperature. Such a film could then be deposited as a counter electrode in SIS junctions without causing any thermal or mechanical degradation to the underlying delicate tunneling barrier. Another advantage of room temperature deposition of NbN is that the substrate could also be polymeric or coated with photoresist making the films accessible for conventional lithographic techniques of patterning.
The occurrence of superconductivity in transition metal nitrides depends sensitively on their stoichiometry and crystal structure. [L. E. Toth, Transition Metal Carbides and Nitrides, Academic, New York (1971), 217] Reactive magnetron sputtering is one of the most suitable techniques for deposition of such materials with stringent composition/structure requirements. Reactive magnetron sputtering offers an excellent control over the rates and pressure of reactants taking part in the reaction, and thereby the stoichiometry of the product. Moreover, the sputtering gas pressure used in the process enables one to control the film microstructure, purity, and stress density in the films; and absence of the secondary electron bombardment of the substrates allows independent control of the substrate temperature, a critical parameter in the deposition of such materials as NbN.
In addition, due to the high kinetic energy bombardment processes involved in sputter deposition, it is generally quite successful in obtaining metastable phases at relatively low substrate temperatures. D. Bacon, et al., J. Appl. Phys. 54 6509 (1983) have used reactive magnetron sputtering on ambient temperature (.about.90.degree. C.) substrates to yield T.sub.c values .about.14.2.degree. K. by optimizing the total sputtering pressure and Ar to N.sub.2 ratio. A totally different approach has been taken by some workers, of including methane [E. Cukauskas, J. Appl. Phys. 54 1013 (1983) and U.S. Pat. No. 4,426,268; E. Cukauskas, et al., J. Appl. Phys. 57, 2538 (1985)] or cyanogen [T. L. Francavilla, et al., IEEE Trans Magn. MAG-17, 569 (1981)], in the reactive gas mixture with the intent of introducing carbon to stabilize the desired B1 (fcc, NaCl type) crystal structure. The lowest substrate temperature used in such a process, as reported more recently by E. Cukauskas, et al., J. Appl. Phys. 57, 2538 (1985), has been 200.degree. C.